Newly discovered Late Triassic Baqing eclogite in central Tibet indicates an anticlockwise West–East Qiangtang collision

The Triassic eclogite-bearing central Qiangtang metamorphic belt (CQMB) in the northern Tibetan Plateau has been debated whether it is a metamorphic core complex underthrust from the Jinsha Paleo-Tethys or an in-situ Shuanghu suture. The CQMB is thus a key issue to elucidate the crustal architecture of the northern Tibetan Plateau, the tectonics of the eastern Tethys, and the petrogenesis of Cenozoic high-K magmatism. We here report the newly discovered Baqing eclogite along the eastern extension of the CQMB near the Baqing town, central Tibet. These eclogites are characterized by the garnet + omphacite + rutile + phengite + quartz assemblages. Primary eclogite-facies metamorphic pressure–temperature estimates yield consistent minimum pressure of 25 ± 1 kbar at 730 ± 60 °C. U–Pb dating on zircons that contain inclusions (garnet + omphacite + rutile + phengite) gave eclogite-facies metamorphic ages of 223 Ma. The geochemical continental crustal signature and the presence of Paleozoic cores in the zircons indicate that the Baqing eclogite formed by continental subduction and marks an eastward-younging anticlockwise West–East Qiangtang collision along the Shuanghu suture from the Middle to Late Triassic.

Based on textural relationships, the amphiboles in the Baqing eclogite may be divided into two groups. One group is the amphibole grains included in garnet grains, and the other group is the replaced amphibole grains that were reaction product of garnet or omphacite in the matrix ( Fig. 2a-d,f). According to the amphibole formula of A 0-1 B 2 C 5 T 8 O 22 (OH) 2 19 , both groups of amphibole grains are Ca or Na-Ca amphibole but not Na amphibole (Table S2). The amphibole grains included in garnet are anhedral and small (<200 μm), while the replaced amphibole grains, mainly barroisite (Brs), are subhedral to anhedral with 10 to 1000 μm in diameter (Fig. 2). The amphibole grains included in garnet exhibit relatively lower SiO 2 (42-49 wt.%), higher total FeO (11-19 wt.%), and (Na + K) A (molar ratio of [Na + K] in site A, 0.426-0.728), than those of replaced amphibole grains in the matrix (SiO 2 , 47-55 wt.%; total FeO, 8-13 wt.%; (Na + K) A , 0.138-0.548) (Table S2; Fig. 3e). However, compositions of both amphibole types overlap each other and are mainly barroisite (Table S2; Fig. 2).
Narrow phengite laths occur in the matrix between garnet, omphacite or amphibole grains, and as inclusions in garnet grains and exhibit Si values of 3.30-3.42 p.f.u. (Table S2; Fig. 2b,c). In general, they appear fresh with no strong deformation. The inclusion phengite grains show lower Si values than the non-inclusion phengite grains (Table S2). Rutile grains occur as inclusion within garnet as well as matrix mineral. They are distinguishable: the former contains more total FeO (~1 wt.%) than the latter (Table S2; Fig. 2a-e). The rutile is partially replaced by ilmenite or titanite (Fig. 2e). Epidote also occurs as inclusion as well as matrix mineral in the Baqing eclogite sample YA-7-18-40 (Fig. 2a,c), while there is less epidote in the eclogite sample YA-7-18-47 (Fig. 2b,d). The epidote compositions are basically coherent with pistacite contents (Fe 3+ /(Al + Fe 3+ )) ranging from 13 to 18 mol.% (Table S2). Similar to amphibole, the inclusion epidote grains are anhedral and smaller (ca. 20 μm) than the matrix epidote (Fig. 2a,c). Only albite rather than plagioclase occurs as the reaction product of omphacite. It  18 ). Abbreviations: A-type-inclusions from kimberlites, basalts, or ultramafic rocks layers; B-type-bands or lenses in migmatite gneissic terrains; C-type-lenses within alpine-type metamorphic rocks, always coexists with blue schists; Alm-almandine, Grs-grossular, Sps-spessartine, Prp-pyrope. (e) X Mg (Mg/(Mg + Fe 2+ )) vs. Si in formula of amphibole. The arrow means the trend of amphibole evolution from inclusion to matrix. See Table S2 for the data employed.  (Table S3). Hereafter we focus on the immobile elements and ratios of specific element pairs, because only these immobile elements could maintain the protolith characteristics under the extreme metamorphic condition 20 62 . The Baqing eclogite samples mainly plotted in the volcanic arc basalt field, along a trend of subduction-related source variation. The western CQMB eclogite data are from ref. 14 . The crustal data are from ref. 63 . The source variation trends are from ref. 36 . The Tianshan eclogite data are from ref. 35 . Abbreviations: WPT-within plate tholeiites; WPA-within plate alkaline; ICA-island arc calc-alkaline basalts; IAT-island arc tholeiites; LC-lower continental crust; UC-upper continental crust; PM-primordial mantle. See Table S3 S4a) and are characterized by low Th and U contents (0.02-0.4 ppm and 1.89-42.3 ppm, respectively) and Th/U ratios of 0.002-0.015. Their Th/U ratios are far less than 0.1, typical of a metamorphic origin [21][22][23] . Among 22 analyzed spots, seven spots were not taken into account for the concordia and weighted mean age calculations based on unreasonable, negative radiogenic 207 Pb/ 206 Pb and 207 Pb/ 235 U ratios, large age error, or older and discrete age (Table S4; Fig. 5a). Fifteen analyses of the metamorphic zircons yielded a concordia age of 223.2 ± 2.4 Ma (mean square of weighted deviation (MSWD) = 0.07) and a weighted mean 206 Pb/ 238 U age of 224 ± 6 Ma (MSWD = 1.5), but the former has much higher probability (0.79) than the latter (0.12) (Fig. 5a).
The zircons from sample YA-7-18-47 can be divided into two groups in terms of their morphologies, internal structures under cathodoluminescence (CL) and Th contents as well as Th/U ratios (Table S4; Fig. S4b). Group 1 zircons are characterized by rounded-ovoid, grey, patch-like structure under CL images, and have dominantly low  Table S4 for the data employed. Th contents of ≤1 ppm and low Th/U ratios of <0.05 (Table S4), indicative of a metamorphic origin 21,22 . Group 2 zircons are characterized by dark inherited core surrounded by narrow grey, metamorphic growth rim (Fig. S4b). Most of these inherited cores display oscillatory zoning and have high Th contents of >200 ppm and high Th/U ratios of >0.4, typical of an igneous origin 21,22 ; in contrast, the growth rims are similar to Group 1 zircons in CL image, Th content and Th/U ratios (Fig. S4b). Metamorphic ages were determined based on data obtained from the strictly metamorphic Group 1 zircons, as well as from the metamorphic growth rims of Group 2 zircons. Great care was taken to discriminate inherited zircon cores from metamorphic growth rims, so that the analytical spots could be placed well away from the boundaries between the two zones (Fig. S4b). The inherited zircon cores were easily distinguished from their growth rims by distinctly irregular boundaries, possibly generated by corrosion during metamorphic reworking 21 , which truncate internal zoning and separate the subrounded to irregular cores from the growth rims (Fig. S4b). A total of fourteen zircon spots were analyzed, including seven Group 1 metamorphic zircons, and two narrow growth rims and five inherited cores of Group 2 zircons (Fig. S4b). Five inherited cores yielded variable ages, ranging from the Cambrian to Carboniferous (535 to 355 Ma Fig. 5b). Analyzed spot 4.1 from the growth rim of a Group 2 zircon has obviously mixed with the inherited older core, considering its much higher Th content (6 ppm) than other metamorphic zircons or metamorphic growth rim (≤1 ppm) as well as the narrow width of the rim (Fig. S4b). It yielded a much high 206 Pb/ 238 U age of 256 Ma, and thus was ruled out from the age calculation. Seven Group 1 metamorphic zircons and one metamorphic growth rim of Group 2 zircon yielded a concordia age 222.8 ± 3.4 Ma (MSWD = 1.09), and a weighted mean age of 223 ± 11 Ma (MSWD = 2.0) (Fig. 5b,c),. but the former has much higher probability (0.3) than the latter (0.05) (Fig. 5a).
The inclusions of garnet, rutile, phengite and omphacite were identified in most zircons from sample YA-7-18-40 and Group 1 metamorphic zircon rims of sample YA-7-18-47, quartz inclusions were identified in one inherited Group 2 zircon core (spot 9.1, 535 Ma Fig. 6). The zircon inclusion assemblages (garnet, rutile, phengite and omphacite) are similar with eclogite matrix mineral assemblages, implying that these zircons grew during or shortly after eclogitic facies metamorphism.
Considering the probabilities (0.05 and 0.12) of the two weighted mean ages of the eclogite are too low to be reliable while the probabilities of the two concordia ages (≥0.30; Fig. 5) are quite acceptable 24 , we use the concordia ages of the two eclogites to define the metamorphic age of the Baqing eclogite. The highly concordant relationships between these two age data subsets (223.2 ± 2.4 Ma and 222.8 ± 3.4 Ma) indicate the ages of 223 ± 3 Ma reliably represent the eclogite-phase metamorphic age of the Baqing eclogite.
The omphacite close to the corresponding garnet and of high Jadeite contents, the garnet rim of high pyrope contents (not the outmost rim), and the phengite of high Si contents in the matrix are chosen to mark the peak metamorphic condition. The Grt-Cpx geothermometer 25 and Grt-Cpx-Ph geobarometer 26 are appropriate for estimates of the peak metamorphic P-T conditions. These two thermobarometers are based on Grt-Cpx Fe 2+ -Mg cation exchange, and thus exact calibration of Fe 2+ content is important. We take total Fe in garnet as Fe 2+ in the calibration since the temperatures estimated by total Fe are indistinguishable to those by Fe 2+ obtained by Mossbauer spectrum 27 . Fe 3+ in omphacite is estimated using the charge balance method of ref. 28 . The peak metamorphism is estimated at conditions of 25 ± 1 kbar/730 ± 60 °C (Fig. 7).
The retrograde metamorphism is marked by titanite as corona around rutile, chlorite and epidote that replaced garnet (Fig. 2d,g), and symplectite of amphibole and albite replacing omphacite at the rim (Fig. 2c). The garnet outmost rim shows a little composition variation (Figs 3b, S3b), indicative of a retrograde metamorphic progress. These minerals may originate from the reaction of Omp + Grt + Rt + H 2 O = Amp + Ab + Ep + Ttn, an important boundary reaction between epidote amphibolite and eclogite facies 29 . Based on the retrograde mineral assemblages, Amp + Pl geothermometer 30 and Al-in-Amp barometer 31 were used. The retrograde metamorphic pressure and temperature are estimated to be 7 ± 0.6 kbar at 480 ± 35 °C (Fig. 7).

Discussion
Continental arc-related protolith of the Baqing eclogite. The relatively moderate Mg contents of garnet (X Mg = Mg/(Mg + Fe 2+ )) are from 0.09 to 0.43, on average, 0.34 (Table S2), compared to mantle eclogite (X Mg = 0.78-0.93) 4,32 , imply that the Baqing eclogite is unlikely of mantle origin, consist with the orogenic origin of the C-type eclogite field 18 (Fig. 3c). Moreover, the mantle eclogite surrounded by ultramafic rock always displays high metamorphic temperature and Mg contents 32 . Compared to mantle eclogite 32 , Baqing eclogite is unlikely of mantle origin. Their moderate TiO 2 contents (Table S3) are distinct from the high TiO 2 contents of the western CQMB eclogite with protolith of continental flood basalt or OIB 4,10 . The low TiO 2 contents of the Baqing eclogite are similar to those of normal mid-ocean ridge basalt (N-MORB) and arc-related basalt 33 . Their LREE enrichments (Fig. 4b) indicate that their protoliths should not be depleted or N-MORB, and the negative Nb-Ta anomalies (Fig. 4c) indicate that their protoliths are contaminated by crustal material and thus formed most likely in a continental arc-related environment 33 . It is also supported by the strongly positive Pb and negative Sr anomalies as depleted Sr and enriched Pb is widespread in upper continental crust 34 . These characteristics are similar to the Tianshan eclogite with continental arc origin (Fig. 4b,c). In Hf/3-Th-Ta triangular discrimination diagram, the Baqing eclogite samples were mainly plotted in volcanic arc basalt field, along a trend of subduction-related source variation, which is different from the trend of western CQMB eclogite 34 (Fig. 4d). Importantly,  Table S4) indicates they were inherited zircons with a magmatic origin 21,22 . The disperse ages of zircon cores indicate that they were captured in the process of upper-continental crust contamination. The protoliths of the Baqing eclogite most likely represent continental arc-related basites erupted in active continental margin, similar to the Tianshan eclogite with continental arc affinities 35 , although the formation age of the protoliths is obscure due to limited SHRIMP zircon dating and the lack of zircon in the basaltic protolith.
When the juvenile and hot arc subducted into mantle, the arc-related basites will be metamorphosed into eclogites with high geothermal gradient (hot eclogite) 36 . This may explain the high geothermal gradient (~9 °C/ km) of the Baqing eclogite. Furthermore, amphibole with different chemical compositions coexisting with eclogitic minerals represents different associated geothermal gradients 37,38 , with Na amphibole (e.g. glaucophane) corresponding to cold eclogites, Ca amphibole (e.g. hornblende) to hot eclogites, and Na-Ca amphibole (e.g. barroisite) to Ep-amphibolites. The Ca amphibole of the Baqing eclogite is consistent with high geothermal gradient, distinctly higher than that of the western CQMB eclogite (~6 °C/km) 4,9 . The Triassic continental arc is located in the southern margin of the East Qiangtang subterrane north of the Baqing eclogite-bearing metamorphic belt (Fig. 1b). It has been found that the crustal materials with the overriding East Qiangtang affinity were involved into the Shuanghu Paleo-Tethyan subduction zone and were then exhumed  (Figs 1, S5). Close to north of the Bangong Meso-Tethyan suture zone and along southern margin of West Qiangtang subterrane, exists Jurassic (-Lower Cretaceous) arc magmatism 13,46 (Fig. 1b). This Bangong Meso-Tethyan branch did not close until the Mid-Cretaceous based on radiolarian-bearing ophiolitic fragments and arc-related magmatic records 13,47 . By contrast, however, the eastern CQMB contains Triassic flysch, ophiolite fragments 1,47 and Triassic tectonic schist 16 , and a Permian-Triassic magmatic arc is juxtaposed close to its north in the East Qiangtang subterrane 48 (Fig. 1b). The Baqing eclogite is within the eastern CQMB and can be tectonically and temporally correlated with the western CQMB eclogite, despite of the different protoliths 4,9,14 (Figs 1b, 4d). Furthermore, the high-pressure metamorphic ages (Late Triassic: 223 Ma) of the Baqing eclogite in the eastern CQMB are much older than those (Early-Middle Jurassic: 194-170 Ma) of the Bangong Meso-Tethyan suture zone (Fig. 1b). Apparently, the eclogite-bearing eastern CQMB represents the relict of a Paleo-Tethyan branch that closed during the Late Triassic 10,47 and is most likely correlated with the Shuanghu suture 10 . Therefore, we believe that the Baqing eclogite is not correlated with the Bangong Meso-Tethyan suture but the Shuanghu suture.

Anticlockwise collision between the West and East Qiangtang subterranes along the Shuanghu
Paleo-Tethyan suture. Identification of the Late Triassic Baqing eclogite in this study confirms both the eastern extension of the Late Triassic CQMB and thus the existence of the Shuanghu Paleo-Tethyan suture that separates the West and East Qiangtang subterranes (Fig. 8). To the east of the Baqing eclogite along the eastern CQMB are exposed the Dingqing Triassic ophiolite fragments 49 , bedded Triassic radiolaria-bearing cherts, trench Triassic turbidites 1,47 , and arc-related magmatic rocks 48 , as well as Late Triassic deformation zone 16 , together supporting the existence of the Triassic Shuanghu suture zone (Fig. 1b). The Baqing eclogite thus provides vital constraints on the tectonothermal evolution of the Paleo-Tethys.
Across the CQMB, the collision between the West and East Qiangtang terranes occurred during the Middle Triassic, as evidenced by dating (237-230 Ma) of the western CQMB continental eclogite 4,9,10 (Fig. 8a). The West and East Qiangtang continental collision in eastern Qiangtang, marked by dating of the Baqing eclogite of this study, is ~10 Ma younger than those in western Qiangtang, showing a scissor-like, eastward-younging, anticlockwise collision between these two continental subterranes (Fig. 8). This model is also supported by the 217-Ma Dingqing ophiolites, east of the Baqing area 48 (Figs 1b, 8).

Methods
Mineral chemistry analyses. Mineral compositions and X-ray compositional mapping of garnets were determined using a JOEL JXA 8100 electron microprobe equipped at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing, China. Mineral compositions are operated at 15-kV accelerating voltage, 20-nA beam current, 10-second counting time, and 5-μm electron beam diameter on the minerals. The detection limit is 0.01 wt.% for all the analyzed elements. Synthetic and natural minerals were used as standards (albite (Na), diopside (Si, Ca), periclase (Mg), hematite (Fe), orthoclase (K), rhodonite (Mn), synthetic Cr 2 O 3 (Cr), synthetic TiO 2 (Ti), and synthetic Al 2 O 3 (Al). The program ZAF was used for matrix corrections 50 . In garnet compositional profiles (Fig. 2c,d), inclusions and cracks in garnets were avoided. Results of representative mineral compositions for the Baqing eclogite are shown in Table S2.
Major and trace element analyses. Whole-rock major and trace element analyses were performed at the Modern Analysis Center, Nanjing University, Nanjing, China, and the IGGCAS, respectively. Major element oxides were analyzed on wavelength-dispersive X-ray fluorescence spectrometry (ARL9800+) using fused glass pellets. Analytical precision determined through replicate analyses is better than 0.5%. Trace elements (including REEs) were analyzed using an Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) (Element, Finnigan MAT) with solution methods. The analytical precision determined through replicate analyses is within 5-10% for all trace elements. Results of major and trace elements are shown in Table S3.
Zircon SHRIMP U-Pb geochronology analyses. Zircon grains from Baqing eclogite (samples YA-7-18-47 and YA-7-18-40) were separated using standard heavy liquid and magnetic methods. Photomicrographs of zircon grains under transmitted and reflected light, and cathodoluminescence (CL) images under Hitachi S3000N SEM were obtained at the Beijing SHRIMP Center, China, in order to reveal the internal structures of the grains and to select target sites. The U, Th, and Pb contents of zircons were measured using SHRIMP II at the Beijing SHRIMP Center, China, under standard operating conditions (15 kV accelerating voltage and a 20-nA beam current). The U-Th-Pb ratios and the absolute abundances of U and Th were determined relative to the standard zircons TEMROA and SL13 51 . Measured compositions were corrected for common Pb using non-radiogenic 204 Pb (sample YA-7-18-47) and 208 Pb (sample YA-7-18-40) based on the different genesis of zircons, and an average crustal composition 52 appropriate for the age of the mineral was assumed. Errors on individual analysis are based on counting statistics at one standard deviation (1σ) level. The weighed mean 206 Pb/ 238 U age data are quoted at 95% confidence level 53 . Results of zircon U-Pb dating data are shown in Table S4.

Zircon inclusion analyses.
In order to define that the zircons or zircon rims were growing as peak eclogite facies mineral assemblage formed, zircon inclusions were identified by a laser Raman microspectrophotometer (Renishaw, UK: inVia Reflex) at School of Earth and Space Sciences, Peking University, Beijing, China. It is equipped with a 532 nm DPSS Laser. The Raman spectra are from 100 cm −1 to 1350 cm −1 and the wave-number accuracy is better than 1 cm −1 . The XY and Z resolutions were about 0.5 μm and 2 μm, respectively, with a 2400 lines/mm grating and confocal mode using a Leica 100 × /0.85 micro-objective. The laser spot power on the surface was set to ~2 mW. The temperature in experimental room was about 22 °C.